This invention refers to a method for shortening the closure time of shutters in cinema projectors, as well as to an apparatus for executing the method by reducing the dark phase, with an optical compensation of the film motion in the uncovered area taking place to increase the luminous efficiency factor. In cinema film projection an intermittent film strip drive has established itself in practice. A very wide variety of shutter designs is employed to withdraw the image from the eye of the viewer during a film transport (dark phase) step. To do this a shutter blade covers the image for the duration of the transport and clears the light path (uncovered region or light phase) when the image has come to a stop. The shutter drive is ordinarily coupled by gears to the intermittent drive and runs synchronously.
According to Talbot's law the light/dark exchange is not perceived if the frequency is adequate, and an impression of continuity is created. With the use of dark fields identical in size, light fluctuations are avoided and the impression of a uniform brightness is created.
The dark phases originating from the shutter substantially reduce the luminous efficiency of the projector. In addition to the glass losses, the shutter, with its 50% efficiency (two-blade shutter) is a major cause of light attenuation.
Various methods intended to increase the luminous efficiency are known.
On the one hand the luminous efficiency can be increased by means of different shutter flap geometry and arrangements. Differing geometries and arrangements lead to a faster or slower covering of the image. The dark phases can be slightly affected.
Diaphragms with blades working in opposite directions or oscillating blades are employed. Tests are also being made with serrated diaphragm edges or perforated diaphragm surfaces, which in turn, however, cause diminished contrast.
On the other hand there are methods utilizing a shutter draw. If the path/time diagram for a film transport step is studied it will be found that the film is only moved at a minimal speed at the beginning and at the end of the step.
In a four-part crosswheel the primary stroke takes place within a range of plus to minus 30.degree. relative to a total angle of rotation of 90.degree.. If the shutter blade is reduced in size so that the step beginning and step end are not covered, then the dark phase is diminished and the luminous efficiency factor becomes greater.
The permissible range is given at 2.5% (0.475 mm) of the total film step movement (19 mm) without the side effects being perceived as all too disruptive.
In any event however a reduction in contrast and a blurring of the image are disadvantageous.
Furthermore, a continuous film transport with optical compensation is known. The optimum is theoretically reached when no shutter at all is necessary.
This can be attained with the aid of the optical compensation. Other than that described in the introductory remarks a continuously transported film strip is taken as a basis.
Optical compensators ensure that the beam path follows the picture so that the image projected onto the screen appears to the viewer to be standing still. There are different basic principles for this.
a) compensation by parallel displacement of a lens that follows the transport motion and then snaps back for the next frame. PA1 b) compensation by mirrors that are tilted to provide a beam deflection. PA1 c) parallel displacement by tilting a plane-parallel glass plate. A polygonal prism is usually used for this. By turning the prism the polygon surface is caused to follow the passing image. The image passes by and the next polygon surface takes up the following image.
The realization of the described optical compensations is extremely complicated and requires the greatest mechanical precision to arrive at satisfactory results at all.
The optical compensators are mechanically driven (for example with a cam drive) synchronously to the film movement. The image position, sharpness and illumination are substantially dependent on this.
The increase in luminous efficiency achieved with the elimination of the shutter is nullified by the optical losses of the extensive optical compensators. The optical compensation therefore has gained no practical significance.
A good description of the prior art is to be found in the book Laufbildprojektion, vol. 6, by Herbert Tummel, published by the Springer Verlag, Vienna, New York; in the book Filmprojektoren/Filmprojektion by Kurt Enz, published by the Fotokinoverlag, 1965; and in the book Kinogeratetechnik by Harald Weise, Hanover, published by the C. F. Wintersche Verlagsbuchhandlung Fussen.